EMPLACEMENT MECHANISMS OF THE SYNTECTONIC MCCARTNEY PLUTON IN THE SW MONTANA FOLD-THRUST BELT: STRUCTURAL, MICROSTRUCTURAL, AND AMS STUDIES
Structures in the country rocks are markedly different between the W and E contacts of the pluton. The W region, between the Pioneer batholith and the pluton, underwent metamorphism and ductile deformation while the E region has preserved the shallow-level deformation of the fold-thrust belt. In the W, ductilely deformed rocks prevail for about 3 km from the pluton contact, showing tightly folded quartzite, slate, calc-silicate and marble of the Cretaceous Colorado Group. Stretch clasts in quartzite metaconglomerates indicate a horizontal stretch S=2-3. Quartz c-axes fabrics in the quartzite layers and calcite microfabrics in marble indicate that deformation-recrystallization took place during bulk coaxial flow with a small component of simple shearing.
We propose that the McCartney Mountain pluton intruded during the development of the fold-thrust belt and was fed by conduits from the Pioneer batholith. The pluton grew in part by crack propagation in the Colorado Group to the east and by ductile deformation of the western contact that had been thermally softened. Subhorizontal lineations in both the pluton and the western contact zone are attributed to a process akin to the growth of a horizontal lens; maximum stretch occurs along the lens horizontal circumference. Magmatic and solid-state structures within and around the McCartney Mountain pluton indicate that this pluton grew in a hybrid manner: brittle processes along the eastern margin and ductile strain in the thermally softened western edge. [We thank the UMN 2002 Adv Field Camp class]